The protection of electrical equipment and lines from overloads and especially from electrical surges has a long history. Because of the extent and expense of their lines, the telephone industry has led the way in this technology. A more or less standard lighting arrester housing is shown in U.S. Pat. No. 2,792,471, which also discloses an air gap surge protection unit.
With the change over from electro-mechanical equipment to transistorized and integrated circuit equipment, the need for overload or surge protection has increased. In most cases today, a "gas tube" protector is employed. See, for example, U.S. Pat. No. 4,208,694 to Gilberts.
Gas-filled protectors consist of two or more electrodes sealed in a hermetic enclosure, which is filled with an inert gas at low pressure. If the device is not sealed completely or has microcracks, the protector can have a slow air leak, which will not be rejected even by manufacturers' 100% quality control testing. Eventually, however, the transient breakdown voltage of the protector, which is of great importance, can be increased 400%-500% because of air leakage.
Because of this problem, the industry has gone to the provision of back-up or secondary path discharge (usually air gap) mechanisms. See, for example, U.S. Pat. No. 4,132,915. Often these devices include milar or plastic insulators which define a secondary or back-up air gap. Often, however, these plastics melt and cover the discharge area of back-up mechanisms which causes the pulse breakdown voltage to be higher than expected.
See the article "Test Those Protectors!" by Nerses Nick Yapoujian, the present inventor, which appeared in the Nov. 1, 1983 issue of "Telephone Engineer and Management". Also, articles "Gas Discharge Systems Furnish Circuit Protection", published in "EDN", June 27, 1985, and "Surge Protector Must Be Fast-action and Fail-safe", published in "Electronic Design", Nov. 21, 1985, also written by Nerses Nick Yapoujian.
It should be noted that failure of the protection device in a manner so as to be open circuited or as high resistance results in possible damage to the equipment to be protected. It is, therefore, highly desirable for a protection device to fail short or closed circuit.
Other patents of interest are:
______________________________________ U.S. Pat. No. Patentee Issued ______________________________________ 2,792,471 D. L. Baxter et al. 05/14/1957 3,522,570 O. Wanaselja 08/04/1970 3,543,207 C. J. Kawieck 11/24/1970 3,813,296 D. D. McStrack et al. 05/28/1974 3,968,303 R. C. Harris 07/06/1976 4,158,869 A. Gilberts 06/19/1979 4,208,694 A. Gilberts 06/17/1980 4,241,374 A. Gilberts 12/23/1980 4,314,304 B. Baumbach 02/02/1982 4,317,155 M. Harada et al. 02/23/1982 4,404,234 P. Zuk 09/13/1983 4,407,849 Laird K. S. Hass et al. 10/04/1983 4,410,124 F. Buechel 10/18/1983 4,463,403 M. Fasano 07/31/1984 ______________________________________
Other patents of possible interest are: U.S. Pat. Nos. 3,569,786, issued to T. Kunugi, on 3/10/1971; 3,813,577, issued on 5/28/1974; 3,818,271, issued to B. Baumbach, on 06/18/1974; 3,849,750, issued to B. Baumbach, on 11/19/1974; 3,886,411, issued to M. Klayum, on 05/27/1975; 3,975,664, issued to B. Baumbach, on 08/17/1976; 4,002,952, issued to G. Menninga, on 01/11/1977; 4,013,927, issued to A. Gilberts, on 03/22/1977; 4,128,855, issued to A. Gilberts, on 12/05/1978; 4,132,915, issued to M. Wilms, on 01/02/1979; 4,168,515, issued to B. Baumbach, on 09/18/1979; 4,314,302, issued to B. Baumbach, on 02/02/1982; 4,320,435, issued to R. Jones, on 03/16/1982; 4,321,650, issued to P. DeLuca, on 03/23/1982; 4,321,649, issued to A. Gilberts, on 03/23/1982; 4,325,100, issued to B. Baumbach, on 04/13/1982; 4,351,015, issued to T. Smith, on 09/21/1982; 4,394,704, issued to R. Jones, on 07/19/1983; 4,405,967, issued to C. Cwirzen, on 09/20/1983.